This invention relates generally to a plasma arc torch for cutting thick metal work plates and, more particularly, to an improved nozzle for directing the plasma stream of the torch toward the plate.
Plasma arc torches for cutting thick work plates are well known as disclosed, for example, by Scott U.S. Pat. No. 4,338,507. In general, a plasma arc torch comprises a body having an electrode adapted to be connected to a voltage source and adapted to direct an electric arc downwardly toward the workpiece. A pressurized flow of oxygen-containing gas such as air is directed past the electrode and into the arc. The gas is ionized by the arc and creates an extremely hot plasma stream for cutting through the workpiece.
Located below the electrode is a nozzle formed with a passage which shapes the plasma stream and increases its velocity. The discharge passages of the nozzles of commercially available plasma arc torches include a converging entrance portion, a rather long cylindrical intermediate portion and a diverging exit portion defined by a pair of axially superimposed chamfers. Because of the shape of the nozzle passage, the plasma stream is discharged from the nozzle at subsonic velocity. Shock waves cause the stream to diffuse and, as a result, the cut at the top of the workpiece on both sides of the kerf is enlarged and gradually converges downwardly before the cut becomes square. Thus, it is difficult to make a right-angled cut along the edge of the workpiece.